Safety is an important concern in every body of water, whether natural or manmade, in which humans swim. Lifeguards are the most commonly used protection to prevent drowning or other injuries. However, lifeguards, even when fully alert, can only monitor limited portions of a swimming pool. Moreover, a swimmer can sink beneath the surface of the water without being detected even by an alert lifeguard. Once a person sinks below the surface of the water, it is unlikely that a lifeguard can, without the help of other swimmers, become aware of the submerged person and his location. Many swimming pools lack lifeguards or have lifeguards present only during certain hours. During unguarded swimming, the likelihood that the presence of a submerged swimmer will be detected is very poor.
In recent years, the importance of promptly rescuing a submerged, distressed swimmer has become apparent. The probability that a near drowning victim will survive decreases significantly with the duration of his submersion. For example, some statistics indicate that a swimmer rescued after only one minute of submersion has a 98 percent probability of surviving while submersion for five minutes or more reduces the survival probability to 25 percent. Even survivors of near drownings may suffer permanent brain damage from extended submersion.
Therefore, for effective rescue by lifeguards or other safety personnel, the existence and location of a submerged, distressed swimmer must be promptly determined. However, when a number of swimmers are present in a pool, it is difficult to detect the presence of a single submerged, distressed swimmer with known apparatus. For example, apparatus for detecting the presence of any persons in a pool, such as that disclosed in U.S. Pat. No. 4,747,085 to Dunegan et al., cannot discriminate between ordinary swimmers and a submerged, distressed swimmer.